Method of socketing a pipe

ABSTRACT

A method of socketing a pipe of oriented plastic having an annular groove inside the socket. An end portion (15) of the pipe is pushed onto a cylindrical mandrel element (11) and the region (l 1 ) of the end portion at the opening thereof where the groove is to be located is heated to a temperature above the glass transition temperature of amorphous plastic and the crystalline melting point of crystalline plastic, respectively. The rest of the end portion is maintained at a temperature below the glass transition temperature or the crystalline melting point, respectively. The end portion (15) when said region is at a temperature above the glass transition temperature or the crystalline melting point, respectively, is pushed onto a second mandrel element (13) axially aligned with the first mandrel element the bottom portion of the socket being formed by said second mandrel element (13). The rest of the end portion is expanded to form the bottom portion of the socket. The end portion of the pipe is cooled and is then withdrawn from the mandrel elements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of socketing a pipe oforiented plastic having an annular groove inside the socket. This grooveis provided to receive an elastic gasket for sealing between the socketand a spigot end of another pipe when two pipes are to beinterconnected.

2. Description of Related Art

The common method in forming a socket or bell on a pipe of plastic whichis not oriented, and at the same time providing an annular groove insidethe socket comprises heating of an end portion of the pipe to atemperature substantially above the glass transition temperature ofamorphous plastic or the crystalline melting point of crystallineplastic, respectively, and pushing the heated end portion of the pipeonto a mandrel having a collapsible core for forming said groove. Thecore is in the expanded condition when the end portion is pushed ontothe mandrel. After cooling of the end portion the core is collapsed andthe socketed pipe is withdrawn from the mandrel.

It is also prior art to slip a sealing ring or gasket onto the mandreland to form the groove in the end portion of the pipe over the sealingring which is left in the groove and is withdrawn from the mandreltogether with the pipe.

When the socket shall be formed on a pipe of oriented plastic,particularly of MOPVC (molecular oriented polyvinylchloride) twoproblems are encountered which do not exist in case of pipes ofnon-oriented plastics material, viz.

1) to avoid stress concentration in the plastic material duringsocketing in the region where the groove shall be formed (the materialis more sensitive to cracking around the periphery than normally becausedraw normally is higher in the hoop direction than in the axialdirection), and

2) to control accurately the degree of possible axial shrinkage duringsocketing thus avoiding unnecessary compression that might reduce theelongation at break,

These problems are overcome by the method of the present invention.

SUMMARY OF THE INVENTION

When the oriented plastic is heated much over the glass transitiontemperature or the crystalline melting point, respectively, it willshrink onto the mandrel, which increases the friction between the pipeand the mandrel. The heated material also is softer, and by thecombination of softer material and increased friction the plastic willbe compressed when the heated end portion is pushed onto the mandrel.Heating of the end portion of the pipe is necessary because the forcesrequired to push the end portion over the expanded collapsible core, thesealing ring, or a prefabricated groove core, respectively, will be toohigh if the temperature of the plastic is below the glass transitiontemperature or the crystalline melting point, respectively. However, inthe method of the invention the compression of the plastic is wellcontrolled and limited mainly to the region of the end portion of thepipe wherein the groove inside the socket is to be located, and thus theforces necessary to move the end portion of the pipe over the core orthe sealing ring, respectively, can still be kept at a low andacceptable value. If the pipe is manufactured with axial draw then theheating of the end portion of the pipe will cause axial draw of the pipeto be reduced which provides an increase of the wall thickness in saidregion sufficient to compensate for the hoop streching of the pipe wallnot only where the groove is located but also in the cylindrical part ofthe pipe.

By limiting the heating of the plastic above the glass transitiontemperature or the crystalline melting point, respectively, to theregion wherein the groove inside the socket is to be located the twoproblems mentioned above thus will be overcome.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below reference beingmade to the accompanying drawings disclosing illustrative embodiments ofthe invention and wherein

FIGS. 1-3 and 4A are side views of a mandrel with a pipe shown in axialcross sectional view and disclose successive steps in the method of theinvention in one embodiment thereof,

FIGS. 4B and 4C are fragmentary views of the mandrel in FIGS. 1-3 and 4Adisclosing a modified structure of the mandrel,

FIGS. 5-9 are similar views as FIGS. 1-3 and 4A disclosing successivesteps in the method of the invention in a second embodiment thereof, and

FIGS. 10-14 are similar views as FIGS. 1-3 and 4A disclosing successivesteps in the method of the invention in a third embodiment thereof.

DESCRIPTION OF THE INVENTION

With reference to FIGS. 1-3 and 4A the mandrel used in the embodiment ofthe method of the invention disclosed therein is made of a hard wearresistent material such as steel, or another suitable material as iswell known in the art. The mandrel comprises a conical end element 10forming the tip of the mandrel, a cylindrical first mandrel element 11,a conical transition element 12, and a second cylindrical mandrelelement 13. The mandrel can have vacuum passages and means (not shown)for heating and/or cooling the mandrel as may be necessary. On mandrelelement 13 there is provided a collapsible core 14 which can be of aconstruction well known in the art. The collapsible core can be adjustedby means not shown between an expanded condition shown i FIGS. 1-3,wherein the core forms an annular ridge the shape of which correspondsto the shape of the annular groove to be provided inside the pipe socketor bell to be formed on the mandrel, and a collapsed condition shown inFIG. 4A, wherein the core is flush with the cylindrical surface ofmandrel element 13. Mandrel element 13 and transition element 12 definethe shape of the socket to be formed.

A pipe of plastic the end portion of which is shown at 15 has uniformcylindrical shape and shall be formed with a socket or bell in said endportion and with an annular groove in the inside surface of the socketfor the reception of an elastic gasket, usually a rubber gasket, whichshall form a seal between the socket and the spigot end of another pipeinserted into the socket. As an initial step of the method of theinvention the pipe end portion 15 may be preheated to a temperaturewhich is close to but still below a temperature T₀ which is the glasstransition temperature T_(g) of an amorphous plastic material, or thecrystalline melting point T_(m) of a crystalline plastic material,respectively, although such preheating is not necessary in practisingthe method of the invention. After preheating the pipe end portion 15 ispushed onto mandrel element 11 which has a diameter substantiallycorresponding to the inside diameter of the pipe, FIG. 1. The outersurface of the mandrel can be made smooth and slippery e. g. by achrome-tetrafluoro ethylene coating. Additionally, a lubricant such assilicone oil can be applied to the surface of the mandrel to adjust thefriction between the pipe and the mandrel when the pipe is pushed ontothe mandrel A length l₁ of the pipe end portion at the opening of thepipe is heated to a temperature T₁ above temperature T₀ when the pipeend portion is on mandrel element 11. This heating can be effected byhot water spray, by radiation heating, or by circulating hot air in ajacket surrounding the pipe. However, the best result would be obtainedby IR heating since the heating effect in that case easily can befocused on a predetermined region of the pipe end Normal IR radiationcomprises wave lengths corresponding to absorption peaks in thematerial, which causes overheating of the outside surface of the pipe.However, if these wave lengths are shut out by a filter e. g. a quartzglass filter, uniform heating of the length l₁ of the pipe end portioncan be obtained as is important in order to control the orientation ofthe material. For example, for poyethylene material the wavelengths tobe filtered away are in the region of 2 to 10 μm, and the effectiveheating wavelength should be substantially 1.2 μm. The surface of theheated portion of the pipe may be protected by a protective gas. Therest of the end portion of the pipe is kept below the temperature T₀.Temperature T₁, should be below the rupture temperature of the plastic,i.e. the temperature at which further temperature increase involvesincreasing risk of rupture of the pipe.

The next step in the method comprises pushing of the end portion of thepipe further axially along the mandrel onto mandrel element 13 viatransition element 12 the end portion of the pipe being pressed overexpanded core 14 to the position disclosed in FIG. 2, As will be seen inFIG. 2 length l₁ is dimensioned such that it extends from the opening ofthe pipe symmetrically at both sides of core 14 so that the groove inthe inner surface of the socket will be formed in that length of thepipe. The soft plastic of length l₁ will be stretched in the hoopdirection over core 14 and may be slightly compressed axially by theforce necessary to move the pipe over the core. By the heating of theend portion of the pipe the possible axial draw of the pipe will bereduced, which increases the wall thickness sufficiently to compensatefor the decrease of the wall thickness which is due to the stretching inthe hoop direction. Selected surfaces of the mandrel may be rough orserrated in order to increase the friction between the mandrel and thepipe. The roughened or serrated surface in combination with vacuum inthe mandrel and/or pressure in a space defined around the mandrel andthe pipe thereon provide means for controlling axial shrinkage of thepipe. With the pipe in this position on the mandrel a length l₂. of thepipe end portion, FIG. 3, may be heated to a temperature T₂ which isabove T₀ and can be but is not necessarily the same temperature as T₁.By this heating which also can be effected by radiation heating in ajacket surrounding the pipe the shape of the length l₂ of the pipe willsnugly conform with the shape of core element 13, transition element 12and core 14, i.e. this length of the pipe will form the grooved socketand the pipe wall may resume the same wall thickness as the rest of thepipe due to the memory of the plastic. It is possible in this stage touse a pressure chamber, vacuum, or mechanical clamps in order to pressthe pipe against the mandrel surface under full calibration of thesocket and the inside groove thereof as is well known in the art.

After cooling of the end portion of the pipe e. g. by means ofsprinkling water, core 14 is collapsed, FIG. 4A, and then the socketedpipe is withdrawn from the mandrel. If the plastic of the pipe isoriented polyvinylchloride (MOPVC), temperature T₀ is about 80° C. andtemperatures T₁ and T₂ can have the same value or can have differentvalues but should be at most 105° C. which is the rupture temperature ofMOPVC. Preferably T₁ is 95° C. and T₂ is 90° C. Correspondingtemperatures for PEX can be T₀ =135° C., T₁ =150° C., and T₂ =140° C.but not higher than 190° C.

Instead of using a collapsible core as described above the groove can beformed over a sealing ring which is passed onto the mandrel and iswithdrawn from the mandrel together with the pipe, the sealing ring thusforming an integral part of the pipe. In this case the depth of thegroove can be reduced and the radius of curvature at the bottom of thegroove can be increased as compared with the bottom radius of a grooveprovided to receive a separate sealing ring or gasket, which means thatstress concentrations in the material will be reduced and that the forcenecessary to push the pipe over the sealing ring on the mandrel will bereduced. Particularly useful in this connection is a gasket of the typehaving a steel or plastic retainer ring which makes possible further toreduce the internal diameter of the groove without the risk of thegasket slipping out of the groove accidentally. The best profile for agasket of this type would be symmetrical with a low angle side rise, i.e. less than 30°, in order that the pipe can be more easily pushed overthe gasket. Also a very large radius of curvature on the top of thegasket or even a straight top will minimize the stresses acting againstthe socket groove. In this connection reference is made to SE-B-463 329the disclosure of which is included herein by reference. Thispublication discloses the use of a separate core having a preformedgroove for a sealing ring therein, said core being slipped onto themandrel. The pipe is pusched over the core which is left in the pipewhen it is withdrawn from the mandrel. This feature is particularly wellsuited to be applied in the method of the invention due to the radialshrinkage of the orientation, which effectively maintains the core inplace. This arrangement known in the field of light weight sewer pipesoffers a new interesting possibility in the field of connecting orientedpolyolefine pressure pipes.

In order to reduce the compression of the pipe when it is pushed ontothe mandrel the arrangement disclosed in FIGS. 4B and 4C can be used. Asshown in these figures the mandrel element comprises segments 13'between the transition element 12 and the core 14 (or a sealing ringslipped onto the mandrel), which are pivotally mounted at the right endsthereof as seen in the figures and can be raised e. g. by hydraulicmeans so that their left ends will be substantially at the same level asor at a higher level than the top of the core 14 (sealing ring), as seenin FIG. 4B. In this position the segments form an ascending path for thepipe 15 when it is pushed onto the mandrel so that the pipe is gentlyexpanded substantially to an inner diameter corresponding to thediameter of the core (sealing ring) and the force necessary in order topush the pipe over the core (sealing ring) will be substantiallyreduced. Then, when the pipe has reached the prescribed position on themandrel the segmnte 13' are shut down to the position in FIG. 4C.

Turning now to the embodiment of FIGS. 5-9 two different mandrels areused when working the second embodiment of the method of the invention.A first mandrel A is shown in FIGS. 5 and 6 and comprises mandrelelements 10A to 13A corresponding to elements 10 to 13 in the embodimentdisclosed in FIGS. 1-3 and 4A. However, core 14 is replaced by acylindrical enlargement 16A of mandrel element 13A, forming a shoulder17A at the left end thereof and joining mandrel element 13A at a conicaltransition 18A at the right end thereof. Enlargement 16A has a diameterwhich corresponds to the diameter of the groove to be formed at thebottom thereof. The second mandrel B is identical with the mandreldisclosed in FIGS. 1-3 and 4A, the elements thereof being numbered10B-14B.

The first step of the method of the invention according to the secondembodiment is shown in FIG. 5 and is identical with the step describedwith reference to FIG. 1 though it is performed on a mandrel ofdifferent shape, viz. mandrel A and more particularly on mandrel element11A. When the length l₁ has been heated to temperature T₁ which is aboveT₀ it is displaced axially on mandrel A to be pushed onto mandrelelement 13A via transition element 12A och over enlargement 16A viatransition element 18A to the position shown in FIG. 6. Length l₁ at theopening of the pipe thus will be expanded to the diameter of enlargement16A, i.e. the maximum diameter of core 14, FIGS. 1-3. The end portion ofthe pipe is cooled when it is still on mandrel A, e.g. by sprinklingwater, and is then withdrawn from mandrel A.

The pipe thus preformed is then pushed onto mandrel B to the positiondisclosed in FIG. 7 and is reheated over the length l₂ to temperature T₂which is above temperature T₀ Then the plastic of the pipe will shrinkonto the mandrel, and the shape of the socket with the inside groovewill be imparted to length l₂, FIG 8. After cooling and collapsing ofcore 14B the socketed pipe will be withdrawn from mandrel B in themanner described with reference to FIGS. 3 and 4A.

The second embodiment of the invention involves an additional step inrelation to the first embodiment, viz. the preforming of the end portionof the pipe but on the other side the expansion of the pipe in the stepof FIG. 6 may require a lower axial force to be exerted on the pipewhich may result in improved quality of the pipe. The embodiment ofFIGS. 10-14 can be seen as a modification of the embodiment of FIGS. 1-3and 4A, wherein the first mandrel element 10 has shorter axial lengthand is supplemented with an expandible sleeve 19 located adjacent theconical transition element 12. Sleeve 19 is made of an elastic materialand can comprise e.g. a rubber sleeve with axial reinforcements, and itcan be expanded by means of pressurized fluid. As disclosed in FIG. 11wherein sleeve 19 is expanded said sleeve in the expanded state thereofcomprises a cylindrical central portion 19A having a diametercorresponding to the diameter of the collapsible core 14, and conicalend portions 19B and 19C adjacent mandrel element 11 and transitionportion 12, respectively. However, central portion 19A can also beconically widening slightly from end portion 19B to end portion 19C forreasons to be explained.

In the step shown in FIG. 10 sleeve 19 is collapsed and the outsidediameter thereof conforms with the outside diameter of mandrel element11 so that the core is flush with said mandrel element. Pipe 15 ispushed onto mandrel element 11 and sleeve 19 as shown in FIG. 10 and isheated to temperature T₁ over length l₁ said length being located oversleeve 19 which at the same time is under internal pressure by means ofpressurized fluid. When the plastic material softens during heatingsleeve 19 will expand and will in turn expand pipe 15 to conform withthe shape of sleeve 19. If sleeve 19 is conical and the surface thereofis made slippery controlled axial compression can be applied to theplastic material of the pipe when the pipe is expanded on sleeve 19. Thepressure of sleeve 19 is released and the pipe is pushed further axiallyalong the mandrel over mandrel element 13 via transition element 12 theend portion of the pipe being pressed over expanded core 14 to theposition shown in FIG. 12. The next step, FIG. 13, corresponds to thatof FIG. 3, and then the procedure terminates by the step of FIG. 14,which corresponds to that of FIG. 4A.

What is claimed is:
 1. A method of socketing a pipe of oriented plastichaving an annular groove inside the socket, characterized inthat an endportion (15) of the pipe is pushed onto a first mandrel element (11;11A) which is cylindrical and has a diameter corresponding to the insidediameter of the pipe, that the end portion is heated to a temperature(T₁) which is above the glass transition temperature (T_(g)) ofamorphous plastic and is above the crystalline melting point (T_(m)) ofcrystalline plastic, respectively, on said cylindrical mandrel elementin the region (l₁) of the end portion of the pipe where the groove is tobe located said region being located at the opening of said end portionof the pipe, the rest of said end portion of the pipe being maintainedat a temperature below the glass transition temperature or thecrystalline melting point, respectively, that the end portion (15) whensaid region is at a temperature above the glass transition temperatureor the crystalline melting point, respectively, is pushed onto a secondmandrel element (13; 13A) axially aligned with said first mandrelelement the bottom portion of the socket being formed by said secondmandrel element (13; 13A), and that the groove is formed by means of acore (14) in said region of the expanded end portion when being at atemperature above the glass transition temperature or the crystallinemelting point, respectively.
 2. The method of claim 1, characterized inthat the groove is formed by means of a collapsible core (14).
 3. Themethod of claim 1, characterized in that the groove is formed by meansof a sealing ring slipped onto said second mandrel element (13; 13A) toform said core, said sealing ring being left in the groove formed whensaid end portion (15) is withdrawn from said second mandrel element. 4.The method of claim 1, characterized in that the groove is formed bymeans of a separate core slipped onto said second mandrel and having apreformed groove therein, said core being left in the pipe when said endportion is withdradwn from said second mandrel element to receive asealing ring in the preformed groove therein.
 5. The method of any ofclaims 1 to 4, characterized in that the end portion (15), when saidregion is at a temperature above the glass transition temperature or thecrystalline melting point, respectively, is expanded to an insidediameter corresponding to the inside diameter at the bottom of thegroove to be formed.
 6. The method of claim 5 characterized in that thegroove is formed in said region of the end portion when this is locatedon said second mandrel element (13) provided with the core (14).
 7. Themethod of claim 6, characterized in that the entire socket after havingbeen pushed over said second mandrel element (13) is heated to atemperature above the glass transition temperature (T_(g)) or thecrystalline melting point (T_(m)), respectively, to conform by shrinkingwith the shape of said second mandrel element.
 8. The method of any ofclaims 5 to 7, characterized in that said end portion of the pipe iscooled and is then withdrawn from the first and second mandrel elements(11,11A; 13,13A).
 9. The method of claim 6 characterized in that saidend portion (15) after having been withdrawn from said first and secondmandrel elements (11A, 13A) is pushed onto a third mandrel element (13B)and is reheated to a temperature above the glass transition temperature(T_(g)) or the crystalline melting point (T_(m)), respectively, toconform by shrinking to the shape of said third mandrel element forforming the remaining part of the socket on said third mandrel elementhaving a collapsible core (14B) for forming said groove inside thesocket, the end portion being cooled and then withdrawn from said thirdmandrel element.
 10. The method of claim 6 characterized in that the endportion (15) is expanded during heating on said first mandrel element(11).
 11. The method of claim 10 characterized in that the end portion(15) is expanded by means of a pressurized sleeve (19) on said firstmandrel element.
 12. The method of claim 11 characterized in that theend portion is expanded on said sleeve (19) to form a conical socket.13. The method of any of claims 1 to 12, characterized in that said endportion (15) of the pipe before being pushed onto said first mandrelelement (11; 11A) is heated to a temperature close to but still belowthe glass transition temperature (T_(g)).
 14. The method of any ofclaims 1 to 13 characterized in that the plastic of the pipe ispolyvinylchloride.
 15. The method of claim 14 characterized in that anytemperature above the glass transition temperature (T_(g)) or thecrystalline melting point, respectively, is at most 105° C.
 16. Themethod of any of claims 1 to 13 characterized in that the plastic of thepipe is crosslinked polyethylene.
 17. The method of claim 16characterized in that any temperature above the glass transitiontemperature (T_(g)) or the crystalline melting point (T_(m)),respectively, is at most 190° C.
 18. The method of any of claims 1 to 17characterized in that said end portion of the pipe is pushed over saidcore (14) on said second mandrel element (13) along a collapsible path(13') gradually ascending to the top of the core in an expanded state,said path then being collapsed when the end portion is positioned onsaid second mandrel element.
 19. The method of any of claims 1 to 18,characterized in that heating of the pipe is effected by hot fluidspray, IR radiation, or circulating hot gas.
 20. The method of claim 19characterized in that the IR radiation when used for heating the pipehas a wave length which deviates from the wavelengths at which thematerial of the pipe has IR absorption peaks.